Dragging Brakes Diagnosis
10-14-2008, 04:59 PM
#16
Drifting
I've got S4 calipers on my '85 and I've had a very similar problem. It has only happened on mine once or twice, after a very long drive, and it seems to affect the rear brakes only. I actually burned up a set of pads over the summer when it happened. But like you had mentioned, the pedal seemed rock hard, like it wouldn't budge at all. It was as if it had lost all vacuum assist. I wondered if the problem isn't related to vacuum. I haven't been able to get the problem to recreate itself to investigate it further.
10-14-2008, 05:50 PM
#18
Burning Brakes
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10-14-2008, 07:07 PM
#19
Burning Brakes
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Update, I recreated the problem, pulled into the garage with the breaks smoking and the engine temp close to the top white mark. I jacked up the front right wheel and confirmed I couldn't move it. Then I loosened up the MC and the brakes freed up. So what does this mean, is my MC ok, but the outer rod needs to be turned in? The lockup occured after a number of hard brakings when my fluid started warming up.
I actually got it to do it twice, the first time I decided I was to far away to drive back like this, so I yanked back on the brake pedal and it instantly freed up.
I actually got it to do it twice, the first time I decided I was to far away to drive back like this, so I yanked back on the brake pedal and it instantly freed up.
10-14-2008, 08:31 PM
#21
Burning Brakes
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10-14-2008, 10:49 PM
#22
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I just went through this myself. I also have a 1986.5. About two years ago I was driving down the
freeway and the car started slowing down even though I kept giving it more gas. I thought the engine
had a problem until I shifted into neutral and the car was stopping! I barely got of an exit ramp and
parked the car. All 4 rotors were pretty warm. I waited about an hour and took the side streets back
home (~17 miles). There was some dragging but I did make it. The next day I flushed the entire brake
system with ATE Gold fluid using a pressure bleeder (Mighty something). The fluid was fairly dark.
I took it for a ride and everything seemed fine. I took longer and longer rides as time went by and I was
absolutely convinced that flushing the brake system had solved the problem. WRONG! Two years later (about 6K miles worth) it came back. It was an exceptionally hot day and I was on the freeway again.
Anyway, I could only drive for about 3-4 miles and the brakes would lock so tightly that the car wouldn't
move. The pedal was tight. I did get the car back home but it took me 5 hrs because of all the waiting
I had to do while I let the brakes cool. I thought for sure that the adjusting link between the booster and
the brake pedal was too long and that by shortening it it would solve the problem. It helped a little but it
wasn't the answer. I then read on Rennlist (yea!) that it could be the booster gone bad (see Tech area, Section 3, dragging brakes Jim Bailey of 928 International wrote). I was highly doubtful that the booster was bad.
After all, there is nothing to them except for a big spring, rubber disc and a sealed chamber. This booster replacement looked like a nightmare to do so I didn't want to put in a used one. I do have a happy ending. I put in a brand new brake booster, bled the brakes a few times and the brakes work beautifully! The threaded rod was put in with just a little play like the book calls out. I also noticed when I replaced the booster - there was still a lot of vacuum in the booster chamber when I removed the big vacuum hose. The car sat for two weeks before I removed the booster too. I then thought for sure that it wasn't the booster (because it had vacuum) and I just wasted $330.00 and two days of my time. Anyway, my car works great and I hope this helps you and anyone else that has a problem like this. I hear that dragging brakes is fairly common. It probably was a sticking brake booster as the new one was much smoother. I aslo want to thank everyone on Rennlist.
freeway and the car started slowing down even though I kept giving it more gas. I thought the engine
had a problem until I shifted into neutral and the car was stopping! I barely got of an exit ramp and
parked the car. All 4 rotors were pretty warm. I waited about an hour and took the side streets back
home (~17 miles). There was some dragging but I did make it. The next day I flushed the entire brake
system with ATE Gold fluid using a pressure bleeder (Mighty something). The fluid was fairly dark.
I took it for a ride and everything seemed fine. I took longer and longer rides as time went by and I was
absolutely convinced that flushing the brake system had solved the problem. WRONG! Two years later (about 6K miles worth) it came back. It was an exceptionally hot day and I was on the freeway again.
Anyway, I could only drive for about 3-4 miles and the brakes would lock so tightly that the car wouldn't
move. The pedal was tight. I did get the car back home but it took me 5 hrs because of all the waiting
I had to do while I let the brakes cool. I thought for sure that the adjusting link between the booster and
the brake pedal was too long and that by shortening it it would solve the problem. It helped a little but it
wasn't the answer. I then read on Rennlist (yea!) that it could be the booster gone bad (see Tech area, Section 3, dragging brakes Jim Bailey of 928 International wrote). I was highly doubtful that the booster was bad.
After all, there is nothing to them except for a big spring, rubber disc and a sealed chamber. This booster replacement looked like a nightmare to do so I didn't want to put in a used one. I do have a happy ending. I put in a brand new brake booster, bled the brakes a few times and the brakes work beautifully! The threaded rod was put in with just a little play like the book calls out. I also noticed when I replaced the booster - there was still a lot of vacuum in the booster chamber when I removed the big vacuum hose. The car sat for two weeks before I removed the booster too. I then thought for sure that it wasn't the booster (because it had vacuum) and I just wasted $330.00 and two days of my time. Anyway, my car works great and I hope this helps you and anyone else that has a problem like this. I hear that dragging brakes is fairly common. It probably was a sticking brake booster as the new one was much smoother. I aslo want to thank everyone on Rennlist.
Last edited by PCARUSA; 10-14-2008 at 11:32 PM. Reason: Added some info. Found article in Tech area.
10-15-2008, 01:24 AM
#23
Rennlist Member
I think i told my 5 year story of the same thing with my bmw. long story short, the Master cylinder was bad. old one looked fine, but didnt allow fluid to return. problem was very intermittant.
new master, solved the issue.
mk
new master, solved the issue.
mk
10-15-2008, 02:40 AM
#24
Nordschleife Master
Mark, for the sake of debating.
If he did have the same issue as you did, then pulling up on the pedal would not bring the brakes back into working order.
Stolar,
There is only 1 nut on there and then the rod threads into a block with the hole through it which allows it to connect to the pedal. Back the nut off, then thread the rod a couple turns into the block.
If he did have the same issue as you did, then pulling up on the pedal would not bring the brakes back into working order.
Stolar,
There is only 1 nut on there and then the rod threads into a block with the hole through it which allows it to connect to the pedal. Back the nut off, then thread the rod a couple turns into the block.
10-15-2008, 09:41 AM
#25
Burning Brakes
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I will give the rod adjustment a shot that sounds like the easiest.
This car doesn't seem to appreciate any maintenance or fixes I do to it. I just replaced everything in A/C system, charged it, pulled the high side hose off the new drier and the schrader decides to stick and blow out all of my refrigerant. I replace the brake hoses and flush the fluid and the MC goes. I just washed the engine bay and the intake temp sensor went, turning my aux fan on during the night draining my battery. So the booster now dying would probably be in line. My 968 seems to be more appreciative of new parts and fixes.
This car doesn't seem to appreciate any maintenance or fixes I do to it. I just replaced everything in A/C system, charged it, pulled the high side hose off the new drier and the schrader decides to stick and blow out all of my refrigerant. I replace the brake hoses and flush the fluid and the MC goes. I just washed the engine bay and the intake temp sensor went, turning my aux fan on during the night draining my battery. So the booster now dying would probably be in line. My 968 seems to be more appreciative of new parts and fixes.
10-15-2008, 01:51 PM
#26
Rennlist Member
actually, that was one of the things i was doing to relieve pressure on the calipers. It sometimes worked and others, it didnt. sometimes it would drag and get so hot that it felt like warped rotors and the sterring wheel would move side to side violently. I would then push the brakes hard, (emergency stops) for several cycles until the brakes got SUPER hot and boiled the brake fluid, and then the problem would go away for a few days. it was very intermittant. again, pulling the pedal back helped. after 5 years of dealing with this in some very unconvential ways, it was the master cylinder.
mk
mk
Mark, for the sake of debating.
If he did have the same issue as you did, then pulling up on the pedal would not bring the brakes back into working order.
Stolar,
There is only 1 nut on there and then the rod threads into a block with the hole through it which allows it to connect to the pedal. Back the nut off, then thread the rod a couple turns into the block.
If he did have the same issue as you did, then pulling up on the pedal would not bring the brakes back into working order.
Stolar,
There is only 1 nut on there and then the rod threads into a block with the hole through it which allows it to connect to the pedal. Back the nut off, then thread the rod a couple turns into the block.
10-16-2008, 01:31 PM
#27
It actually could be either one causing the problem but I would adjust the pedal rod first since it is so much easier to do. Make sure your brake pedal return spring is operational first. If the pedal rod is properly adjusted the clevis pin will slide in with having to move the pedal to align the holes. If you have to push the pedal down to align the holes before you adjust it then the pedal rod is not the problem so I would check the master cylinder rod in the brake booster.
10-16-2008, 01:50 PM
#30
The root cause is something is preventing the fluid from returning to the resivoir. There is a small compensation hole just in front of the master cylinder piston cup seal. As soon as the brake is pushed "the slightest piston movement" the cup seal passes over and seals this compensation hole so pressure can build in the system. So your problem is either the piston is too far forward in the master cylinder bore or you have some foreign material acting as a one way valve in the compensation hole.